There has been a rapid increase in demand of illuminance sensors having properties close to human visual properties, in order to restrain the drain on battery power of mobile phones or increase the visibility of liquid crystal display, by automatically adjusting the brightness of the backlight of mobile phones and liquid crystal television sets in accordance with ambient light.
Also, because of digitalization and advance of illuminance sensors, there has been a demand of easy-to-use and low-cost illuminance sensors for liquid crystal backlight automatic light adjustment systems.
Representative examples of visible light sensors are silicon photodiodes and CdS (cadmium sulfide) cells.
Silicon photodiodes are widely used for optical communications, light receiving elements for optical discs, and optical sensors, because of the small size, high-speed response, and stability.
However, being significantly different from those of humans, the spectral sensitivity characteristics of silicon photodiodes are sensitive to infrared light. To arrange the silicon photodiodes to have the spectral sensitivity characteristics close to those of humans, it is necessary to include a circuit and a visibility correction filter in order to adjust the spectral sensitivity characteristics.
On the other hand, on account of its spectral sensitivity characteristics close to those of humans, CdS cells have long been used as exposure meters of cameras and visible light sensors.
However, the use of CdS cells, which are mainly made of cadmium sulfide, has gradually been restricted these days, in consideration of environmental burdens. Since July 2006, it has been prohibited to bring products using at least one of cadmium, lead, hexavalent chromium, and mercury into Europe. Because of this, there has been an increase in the demand of sensors which are made of environmentally-friendly silicon photodiodes, having spectral sensitivity characteristics close to those of humans.
For example, Japanese Laid-Open Patent Application No. 10-142047 (published on May 29, 1998) teaches as follows: plural photodiodes are included in an illuminance sensor, light entering from a light receiving window is separated by a shielding plate provided between neighboring photodiodes, and illuminance is detected in each of plural regions. Because of this arrangement, the illuminance distribution is precisely detected even if intense light locally enters.
Japanese Laid-Open Patent Application No. 9-145468 (published on Jun. 6, 1997) teaches that illuminance data used for quickly responding to a change in room illuminance is generated from the previously-output illuminance data and detected illuminance data. This arrangement reduces an amount of stored illuminance data, at the same time improve the response to a change in illuminance.
Japanese Laid-Open Patent Application No. 2004-22646 (published on Jan. 22, 2004) teaches that, ambient brightness detected by a photo transistor is obtained as an illuminance level, and white LEDs are driven in accordance with the duty ratio of a PWM signal corresponding to the illuminance level.
Japanese Laid-Open Patent Application No. 2004-233569 (published on Aug. 19, 2004) teaches a technique to circumvent an influence of a noise from an LED power source circuit, when ambient illuminance is detected. In this technique, when illuminance is detected by an illuminance level detection circuit, a sensor power source circuit is turned on while an LED power source circuit is turned off, so that a noise of the LED power source circuit is circumvented.
The above-described arrangements, however, are disadvantageous in that signals from the illuminance sensor are susceptible to noise when illuminance is low, and an apparatus for adjusting light bears a burden of processing the signals from the illuminance sensor.
FIG. 15 is a block diagram showing a conventional light control apparatus. This light control apparatus is arranged such that, to allow an analog output illuminance sensor 510 to output, in accordance with the illuminance, a voltage or current analog signal having spectral sensitivity characteristics close to human visibility, (1) an output of an illuminance sensor is sampled, (2) the output is converted into a digital signal for controlling a light emitting apparatus, such as a PWM signal, and (3) light is controlled by controlling the light emitting apparatus such as LED.
In the conventional art shown in FIG. 15, after converting an analog signal output from the analog output illuminance sensor 510 into a digital signal by an A/D converter, a CPU 520 performs computation so that a PWM signal corresponding to the illuminance is generated. The PWM signal is input to a PWM modulation terminal of a general-purpose LED driver 530, with the result that light from an LED backlight or the like is automatically controlled.
In the scheme shown in FIG. 15, the computing apparatus (CPU 520) is required to always sample outputs from the analog output illuminance sensor 510 so as to perform computation. The CPU 520 must therefore bear a burden, and this may cause an adverse effect on the execution speed of other applications. A CPU dedicated to automatic light control may be additionally provided to avoid such a performance problem of the CPU 520, but this drives up costs.
When the illuminance is low, the output level of the analog output illuminance sensor 510 is also low. Therefore the analog output illuminance sensor 510 is susceptible to noise when a line between the analog output illuminance sensor 510 and the CPU 520 is long.
There is another known scheme shown in FIG. 16, which is arranged such that a digital-output illuminance sensor 510a is adopted and the illuminance sensor and the CPU are connected by a serial interface such as I2C, so that illuminance information is sent and received as digital signals. This scheme is advantageous in that an influence of noise is restrained because illuminance information is transmitted between the digital-output illuminance sensor 510a and the CPU 520, in the form of digital signals.
However, being similar to the scheme shown in FIG. 15, the CPU 520 is required to always monitor the illuminance. Therefore the problem of the burden on the CPU 520 is unsolved. Also in this case, a CPU dedicated to automatic light control may be additionally provided to avoid the performance problem of the CPU 520, but this drives up costs.
Japanese: Laid-Open Patent Application No. 2004-22646 (published on Jan. 22, 2004) teaches that, an LED driver includes an A/D converter or the like and hence analog signals output from an analog output illuminance sensor are converted into digital signals, computation is then suitably carried out and LED currents are adjusted in accordance with the illuminance, so that light from an LED backlight or the like is automatically controlled.
FIG. 17 is a block diagram showing the LED driver disclosed by Japanese Laid-Open Patent Application No. 2004-22646 (published on Jan. 22, 2004). This LED driver can be alternatively represented by the block diagram in FIG. 18.
In the scheme illustrated in FIG. 17 and FIG. 18, a high-performance LED driver 530a includes an A/D converter or the like as discussed above, and hence LED currents are adjusted in accordance with the illuminance so that light is controlled. In this arrangement, the CPU 520 and the high-performance LED driver 530a are connected by a serial interface such as I2C. It is therefore possible to determine the initial setting of the high-performance LED driver 530a, at the time of power on.
In the arrangement shown in FIG. 17 and FIG. 18, the CPU 520 is required to only determine the initial setting of members such as a register of the high-performance LED driver 530a, at the time of power on or reset. The CPU 520 is therefore not required to always sample illuminance information. On this account, the scheme makes it possible to construct an automatic light control system without lowering the performance of the CPU 520.
In the scheme above, however, since the most of the functions concerning the light control are performed by the high-performance LED driver 530a, the high-performance LED driver 530a must be custom-build for each type of the analog output illuminance sensors 510 and each type of light control applications. Because of this lack of versatility, the LED driver is costly.
The scheme is also disadvantageous in that, because the analog output illuminance sensor 510 outputs illuminance information as analog signals, a noise influence is not negligible when the illuminance is low.
To reduce a noise influence on signals from an illuminance sensor, Japanese Laid-Open Patent Application No. 2004-233569 (published on Aug. 19, 2004) teaches that, when an illuminance level detection circuit detects illuminance, a sensor power source circuit is turned on while an LED power source circuit is turned off, in order to avoid an influence of a noise from the LED power source circuit. This scheme, however, is disadvantageous in that light may flicker because power supply to LEDs stops each time the illuminance level is detected.
There is a case where an illuminance sensor is arranged to detect the illuminance of plural areas as taught in Japanese Laid-Open Patent Application No. 10-142047 (published on May 29, 1998). Also in this case, signals from the illuminance sensor are susceptible to a noise when the illuminance is low, and hence an apparatus for light control is required to bear a burden of processing of the signals from the illuminance sensor.